Looped local networks are, for instance, described in the following technical papers:
(1) "An Introduction to Local Area Networks" by David D. Clark & al, published in the "Proceedings of the IEEE", Vol. 66, No. 11, November 1978, pages 1499 and 1500, PA1 (2) "An experimental distributed switching system to handle bursty computer traffic" by W. D. Farmer and E. E. Newhall, published in "Pro. ACM Symposium on Problems in the Optimization of Data Communications", October 1963, pages 31-34, and PA1 (3) "A local-area communication network based on a reliable token-ring system" by W. Bux & al, in "Local Computer Networks", IFIP, 1982, pages 69-82.
Token control of communications in packet mode is also known and described, for instance, in the above mentioned papers (1) and (2).
On the other hand, a communication network between terminals in a data processing system operating in packet mode within a time division multiplex is described in the European Pat. No. 0,036,808 (corresponding to the U.S. patent application Ser. No. 244,170, now U.S. Pat. No. 4,413,338).
It will be recalled that in the following the term "first order multiplex" is more particularly related to a 32-channel TDM multiplex. Such a first order multiplex is described in the booklet published on Sept. 20, 1973, by the "Conference Europeenne des administrations des Postes et Telecommunications", and entitled: Specifications of first order PCM multiplex system". But, it will be understood that nearly related first order multiplex, such as the 24-channel first order multiplex standardized in the USA, may also be used in the network according to the invention.
Each frame of a 32-channel first order multiplex has a duration of 125 microseconds and comprises 32 channel time slots iT0-iT31. Each channel time slot comprises 8 time slot w1-w8, each time slot being used for transmitting a bit 1 or 0. In each frame, the time slot iT0 is assigned to the frame alignment. When channel associated signalling is used in such a multiplex, the frames are grouped in multiframes of sixteen consecutive frames 0-15 each, and the multiframe alignment signal is 0000, that signal occupying the first four time slots w1-w4 of channel time slot iT16 in frame 0. Furthermore, channel time slot iT16 in each frame is used for transmitting the channel associated signalling. The other channel time slots iT1-iT15 and iT17-iT31 are used for usually transmitting the allotted speech samples.
An other multiframe structure is described in the above mentioned European Pat. No. 0,036,808. Each multiframe comprises 20 first order multiplex frames. In each frame, the channel time slot iT0 normally contains the frame synchronization information. In each multiframe, channel time slots iT16 in frames 1-19 are assigned to service information exchanges. In each frame, channel time slots iT1-iT15 and iT17-iT31 are allotted to data exchanges. Thus, the signalling messages for establishing and interrupting a communication, for supervising the system, and so on, which are transmitted through the service channel corresponding to the channel time slot iT16 in each frame, may be varied in size. The choice of a 20-frame multiframe makes available a multiframe alignment used to justify the flow of signalling messages with respect to the frame 0 as far as request messages are transmitted from the control unit toward the access equipments, and with respect to the frame 10 as far as response messages are transmitted from the access equipments toward the control unit. Furthermore, that choice of 20 frames per multiframe makes it possible to multiplex 2400-baud channels, or of course 1200-baud channels, while allowing to transmit a maximum flow of 64 kbit/s, with 48 useful kbit/s to make the network compatible with the envelopes defined in the CCITT .times.50 recommendation. At last, the exchange of signalling messages allows to allot, in each multiframe, the frame(s) and the iT of said frame(s) wherein the information relative to one communication direction is placed.